R-band DNa replicates in the first half of S-phase and G-band DNA replicates in the last half. By timely administration of BrdUrd to synchronized cells followed by metaphase arrest and density gradient fractionation of the chromosomal DNA, the R-band DNA can be separated from the G-band DNA, and their differences characterized. We are nick translating these fractions and driving them to hybridization with total DNA and m-RNA to see if middle repetitive sequences are concentrated in G- or R-bands and to see if m-RNA is more complimentary to G- or R-band DNA.